StateCache

This library supports fast and customisable caching of the public properties of an object. The caching object is built dynamically to only include the properties from the object that are required to be cached. This allows the store/restore to only be maginally slower than manually written caching code.

Warning: The number of places where this technique should be used are few. Simple get/set properties (i.e. acting as a data bucket) on objects typically indicate poor OO design and hence one would expect the number of places where it makes sense to cache them in this manner to be quite small. The extra effort required to manage state in this manner should only be taken if there is a good performance reason or you are interacting with a third-party library that is designed in that manner (hopefully itself for performance reasons).

Motivating Example

There is form of system design in which state is the primary way in which activities are configured. This is often done for performance reasons - you may not want to set all the properties of the system for every activity, so the state from other activities is "reused" and only changes to the specific state values necessary for configuring the activity at hand are changed.

For example a graphics system might draw everything in blue following the setting of the foreground colour to blue. Any call to Draw something will result in a blue entity being drawn until the foreground colour is switched to a different one.

Often a state change is required locally within a drawing object. In order to ensure this state change doesn't impact any objects later in the program flow, the initial value is stored before the local changes are made and restored after the action has been performed.

void Draw(StatefulGraphicsSystem system)
{
    // Draw a blue line
    var oldValue = system.Color;
    system.Color = Color.Blue;
    
    system.DrawLine(....);
    
    system.Color = oldValue;
}

Sometimes a number of state changes are made which all require restoring...

void DoSomething(StatefulSystem system)
{
    var oldValue1 = system.Value1;
    var oldValue2 = system.Value2;
    var oldValue3 = system.Value3;
    
    system.Value1 = ...;
    system.Value2 = ...;
    system.Value3 = ...;
    
    system.DoSomething();
    
    system.Value1 = oldValue1;
    system.Value2 = oldValue2;
    system.Value3 = oldValue3;
}

Typically the state properties that need caching are some subset of the overall set of possible state properties.

Solution

This library changes the code in the motivating examples above to something like the following...

public class DrawableEntity
{
    readonly StateCache<StatefulDrawingSystem> stateCache;
    
    public DrawableEntity()
    {
        stateCache =
            new StateCacheBuilder<StatefulDrawingSystem>()
                .Property("Value1")
                .Property("Value2")
                .Property("Value3")
                .Build();
    }
    
    public void Draw(StatefulGraphicsSystem system)
    {
        using (stateCache.AutoRestoreToPreviousState(system))
        {
            system.Value1 = ...;
            system.Value2 = ...;
            system.Value3 = ...;
            
            system.DrawSomething();
        }
    }
}

Alternatively you can use the library without the "using".

public class DrawableEntity
{
    readonly StateCache<StatefulDrawingSystem> stateCache;
    
    public DrawableEntity()
    {
        stateCache =
            new StateCacheBuilder<StatefulDrawingSystem>()
                .Property("Value1")
                .Property("Value2")
                .Property("Value3")
                .Build();
    }
    
    public void Draw(StatefulGraphicsSystem system)
    {
        stateCache.Store(system);
        
        system.Value1 = ...;
        system.Value2 = ...;
        system.Value3 = ...;
            
        system.DrawSomething();
        
        stateCache.Restore(system);
    }
}

Design

The key requirements of the library are

speed of store/restore close to that of the hand-rolled version
customisability of the properties that are cached
simplicity of usage (both configuration and store/restore)
robustness

In order to deliver both the speed and customisability a design using Reflection.Emit was chosen. The combination of customisability and speed rules out a solution using only C# types - one early proof of concept used a list of Action objects to perform the restore. This was several orders of magnitude slower than both the Reflection.Emit and hand-rolled versions.

Performance

The library comes with BenchmarkDotNet tests that show this only adds a small overhead (from an already small base) to the hand-rolled version. The non-using version runs marginally quicker than the using version.

TODO

License

mbreckon/StateCache

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